Azure subnet, NSG, and private-endpoint patterns for adding subnets inside an existing VNet — atomic NSG-at-creation, per-service delegation, PE subnet layout, ARM write serialization, and reference NSG layouts for PE, APIM, App Gateway, and Container Apps subnets. Tool-agnostic guidance with Terraform (azapi_resource), Bicep, and az CLI examples. Use when adding subnets, debugging delegation or PE errors, or sizing a PE subnet.
Installation
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Azure subnet, NSG, and private-endpoint patterns for adding subnets inside an existing VNet — atomic NSG-at-creation, per-service delegation, PE subnet layout, ARM write serialization, and reference NSG layouts for PE, APIM, App Gateway, and Container Apps subnets. Tool-agnostic guidance with Terraform (azapi_resource), Bicep, and az CLI examples. Use when adding subnets, debugging delegation or PE errors, or sizing a PE subnet.
Azure Networking — Subnets, NSGs, and Private Endpoints
Use When
Creating a new subnet inside an existing Azure VNet with any IaC tool (Terraform, Bicep, ARM, az CLI).
Choosing the right service delegation for a VNet-injected workload (Container Apps, App Service / Functions VNet integration, etc.).
Designing a private-endpoint (PE) subnet and sizing it for the services that will land on it.
Debugging SubnetMustNotHaveDelegation, SubnetDelegationCannotBeChanged, AnotherOperationInProgress, NetworkSecurityGroupNotAssociated, or PrivateEndpointCannotBeCreatedInSubnet.
Laying out NSG inbound/outbound rules for APIM, App Gateway, Container Apps Environment, or PE subnets.
Operating under Azure Policy that requires every subnet to carry an NSG at creation time (e.g., landing-zone policies).
Don't Use When
Provisioning the VNet itself, or designing hub-spoke / landing-zone topology — use the upstream azure-enterprise-infra-planner skill.
Authoring APIM policies, AI gateway behavior, or backend routing — use upstream azure-aigateway.
Authoring App Gateway WAF custom rules or rewrite sets — out of scope.
Provisioning AKS networking (CNI Overlay, private API server) — use upstream azure-kubernetes.
General azd-style app prep that happens to need a subnet — use upstream azure-prepare.
The upstream Microsoft azure-* skills referenced above live in the Microsoft agent-skills catalogue — install separately if not already in your agent environment.
Workflow
Anchor to the BC Gov Azure Landing Zone networking rules first; these rules take precedence. Reference: BC Gov Azure Landing Zone — Networking. These platform constraints override anything below if they conflict:
Platform-protected, do not create or modify — VNets, VNet address space, VNet DNS settings, VNet peerings, ExpressRoute / VPN / NAT / Local Gateways, Route Tables, and the setbypolicy diagnostic setting. Request changes via the Public Cloud Service Request.
IP budget — each Project Set gets ~251 usable IPs (a single /24); Microsoft reserves 5 IPs per subnet; raise a Service Request for more.
NSG-at-creation is mandatory — every subnet must carry an NSG from the moment it is created, and every subnet must be a Private Subnet (defaultOutboundAccess = false, Zero Trust).
Egress — all outbound traffic is forced through the central vWAN hub and firewall; teams do not add their own Internet-egress UDRs.
Spoke-to-spoke is off by default — open a Public Cloud Service Request if you need it.
Prefer Private Endpoints over VNet integration / Service Endpoints — PE DNS records are auto-registered in the centralized Private DNS Zone; teams must not create their own Private DNS Zones.
Confirm the parent VNet exists and reference it as an existing resource (Terraform data.azurerm_virtual_network, Bicep resource ... existing, az CLI az network vnet show) so you can pin to its ID.
Pick CIDR and size — confirm Azure's minimum subnet size for the target service, that the CIDR fits inside one of the VNet's address spaces, and that it does not overlap any existing subnet.
Pick delegation — look up the target service's required serviceDelegations entry (e.g., Microsoft.App/environments for Container Apps; Microsoft.Web/serverFarms for App Service / Functions VNet integration). Many services need no delegation at all (PE subnets, App Gateway, APIM internal mode).
Create the subnet with its NSG attached in the same API call — declare networkSecurityGroup on the subnet body itself, not as a separate association. Tool specifics: Terraform — use azapi_resource with networkSecurityGroup.id inside body.properties (the azurerm_subnet + azurerm_subnet_network_security_group_association pair is two PUTs and leaves an unprotected gap); Bicep/ARM — set the networkSecurityGroup property on the subnet resource; az CLI — pass --network-security-group to az network vnet subnet create.
Serialize sibling-subnet writes — ARM serializes writes against the same VNet; parallel writes fail with AnotherOperationInProgress. Terraform — list every preceding sibling subnet in depends_on. Bicep — rely on the implicit resource graph, or add an explicit dependsOn array when siblings live in separate modules. az CLI / scripts — issue creates sequentially.
(Terraform only) Lock the VNet — add locks = [data.azurerm_virtual_network.target.id] to every subnet's azapi_resource so Terraform's own parallelism doesn't race ARM in the gap between dependent operations. Bicep/ARM deployments don't need this — the deployment engine doesn't parallelize within one scope.
For PE subnets — set privateEndpointNetworkPolicies = "Disabled" (the long-standing default, which means NSGs and UDRs on the subnet are not evaluated for the PE NIC) and no delegation. If you do need to override the PE's /32 default route through a firewall, set "RouteTableEnabled" (or "Enabled" for both NSG and UDR); for defence-in-depth NSG enforcement on a PE NIC itself, use "NetworkSecurityGroupEnabled". Size the CIDR for the worst-case service that will land on it (some services consume more than one IP per PE — see references).
For App Gateway subnets — no delegation. If asymmetric routing is observed and your platform allows custom Route Tables, attach one with 0.0.0.0/0 → Internet (plus any internal corp ranges that must not egress via ExpressRoute / VWAN hub). BC Gov Landing Zone: Route Tables are platform-protected (see Step 1) — raise a Public Cloud Service Request for any required UDR.
Expose subnet ID and CIDR as outputs (Terraform output, Bicep output, deployment script return value) so downstream stacks can read them via terraform_remote_state, Bicep existing references, or another state-sharing mechanism.
Validate before apply — run the tool's validate + dry-run path: Terraform fmt -recursive, validate, plan; Bicep az bicep build then az deployment ... what-if; ARM az deployment ... what-if. Catch CIDR overlap and delegation errors before apply.
Rules
Always create the subnet with its NSG attached in the same API call — not as a separate association step. (Why: NSG-at-creation policies deny the gap between subnet PUT and NSG association. Tool specifics: Terraform — azapi_resource with networkSecurityGroup.id inside body.properties; Bicep/ARM — set networkSecurityGroup on the subnet itself; az CLI — pass --network-security-group to az network vnet subnet create.)
Always serialize sibling-subnet writes against the same VNet. (Why: ARM rejects concurrent subnet PUTs with AnotherOperationInProgress. Tool specifics: Terraform — list every preceding sibling subnet in depends_on; Bicep — rely on the implicit resource graph or use an explicit dependsOn array when siblings are in separate modules; scripts/CLI — create subnets sequentially.)
Never share a delegated subnet between services. (Why: each subnet allows exactly one serviceDelegations entry; the second service errors at associate time.)
Never set a delegation on a Private Endpoint or App Gateway subnet. (Why: Azure rejects PE creation in delegated subnets, and App Gateway refuses to launch in one.)
Never move an existing Private Endpoint to a different subnet without a maintenance window. (Why: moving a PE destroys and recreates it, breaking the customer-facing DNS record until propagation completes.)
Always set privateEndpointNetworkPolicies explicitly on PE subnets — typically "Disabled", which is the long-standing default and is what most BC Gov PE designs use. (Why: this property controls whether the subnet's NSG and UDR are evaluated for PE NIC traffic; "Disabled" skips both, "NetworkSecurityGroupEnabled" applies NSGs only, "RouteTableEnabled" applies UDRs only, "Enabled" applies both. Most threat models for private endpoints only need to govern the consumer side of the connection — the workload subnet talking to the PE — because the PE itself only exposes the upstream Azure service and has no arbitrary egress to control. Pin the value in IaC so behaviour is reproducible across regions and subscriptions where the platform default may have drifted. Switch to "RouteTableEnabled" (or "Enabled") when you need a UDR to override the PE's /32 default route through a firewall or NVA.)
Always size PE subnets for the worst-case service that will land on them. (Why: Cosmos DB PE consumes 2 IPs — one global + one regional endpoint; Azure AI Services "AIServices" kind exposes up to 3 sub-resources = 3 IPs per PE. A /27 fills surprisingly fast.)
Always expose subnet IDs as outputs (Terraform output, Bicep output) even if no downstream stack reads them yet. (Why: adding outputs later is cheap, but a downstream stack that needs the ID can't read state it isn't exposed to.)
Never modify VNet address space or create a Private DNS Zone — both are managed centrally by the platform team (see Step 1's platform-protected list; raise a Public Cloud Service Request instead).
Examples
"Add a Container Apps Environment subnet" → declare the subnet with a Microsoft.App/environments delegation (Terraform azapi_resource body, Bicep delegations array on the subnet, or az network vnet subnet create --delegations Microsoft.App/environments); attach an NSG that allows outbound 443 to AzureContainerRegistry, AzureMonitor, AzureActiveDirectory, and VirtualNetwork; serialize against every other subnet in the VNet.
"Deployment keeps failing intermittently with AnotherOperationInProgress" → sibling subnets are being written in parallel against the same VNet. Terraform — add depends_on = [azapi_resource.first_subnet] to the second, and [azapi_resource.first_subnet, azapi_resource.second_subnet] to a third. Bicep — add dependsOn: [first, second] when siblings span separate modules. CLI — stop backgrounding the az network vnet subnet create calls.
"App Gateway deployment fails with SubnetMustNotHaveDelegation" → remove the delegation from the AppGW subnet (Terraform: drop the delegations block from the azapi_resource body; Bicep: remove the delegations array from the subnet; CLI: omit --delegations).
"PE creation fails with PrivateEndpointCannotBeCreatedInSubnet" → subnet has a delegation incompatible with PEs, or privateEndpointNetworkPolicies is set to a value the PE provider's automation refuses. Set the property to "Disabled" and remove any delegation.
Edge Cases
VNet owned by a separate IaC stack — reference it as an existing resource in the current root (Terraform data.azurerm_virtual_network, Bicep resource ... existing); pin to its ID for the parent and (Terraform only) for locks. Do not try to import the VNet.
NSG drifts on an existing subnet — Terraform azapi_resource reapplies the NSG ID on every plan because the property lives in the subnet body; azurerm_subnet may not surface the drift until the association resource is also reconciled. Bicep redeployments similarly reassert the property since it's declared inline on the subnet.
Peered VNet needs direct ingress to a VNet-injected service without going through App Gateway — add inbound NSG rules on the target subnet at caller-assigned priorities in a reserved band (e.g., 400–499) so adding/removing peers never shifts existing rules.
Delegation needs to change on an existing subnet — Azure returns SubnetDelegationCannotBeChanged. The only fix is to destroy and recreate the subnet (Terraform terraform destroy -target or taint; Bicep — remove from the template and redeploy, then add back; CLI — az network vnet subnet delete then re-create). Schedule the outage.
See references/REFERENCE.md for the atomic subnet+NSG body shape in Terraform, Bicep, and az CLI; full NSG rule tables per subnet type; sibling-subnet serialization examples; App Gateway route-table requirements; PE subnet capacity math; and a failure playbook keyed by Azure error code.
For broader Azure topics, prefer these upstream Microsoft skills (browse the catalogue at https://microsoft.github.io/skills/#agents and install separately if not already in your agent environment) instead of duplicating their guidance here:
VNet, hub-spoke, and landing-zone design → azure-enterprise-infra-planner
